There are already known various solid fuels which are referred to as Refuse Derived Fuel (RDF) or Waste Derived Fuel (WDF). Fuels of this type are disclosed in U.K. Patent documents GB 1551020, GB 1597443, GB-A-2046299, GB-A-2076013. However, the fuels described in the above specifications suffer from one or more of the following disadvantages:
For example, the known processes for RDF/WDF fuels entail the deployment of costly and complex plant, i.e. `ballistic separation plant`, `air classifiers` to separate the light fractions from the non-combustibles (metal, glass) from the general crude refuse. Above all RDF/WDF fuels require substantial dryers with a high energy consumption, because of the inherent high moisture content of general refuse, i.e. often more than 40%, and increasing sharply during spring and summer, as householders for instance dispose of grass cuttings from lawns.
RDF/WDF fuels as described in the above-mentioned documents have a medium calorific value (between approx. 4300 to 7000 BTU/lb (10 000 to 16 300 KJ/kg)). The RDF/WDF fuels also have a high pollution factor due to the presence of plastics and toxic substances contained in general refuse. The RDF/WDF fuels are unsuitable for bulk storage--they readily absorb environmental moisture, and can then be subject to `bio-degration`, i.e. `compost heap-effect` and possible explosion risk if stored in bulk. Such RDF/WDF fuels have only a limited value to the industrial user because of their relatively low calorific value compared to coal, and they are not suitable for burning on open fires in a domestic situation.
Furthermore, RDF/WDF fuels as described in the above-mentioned documents present a possible health risk to operating/maintenance staff, i.e. unpleasant odours, toxic fumes, which can result in skin irritations and more serious side effects to health. The RDF/WDF fuels contribute to boiler corrosion because of the acid elements contained in refuse/waste derived fuels. The adoption of RDF/WDF fuel production as described in the above-mentioned documents is unlikely to be suitable for small local authorities or small companies, because of the high capital investment and high running costs to maintain such an operation. The energy consumption for dryers, separation plant, air classifiers in such an operation for the RDF/WDF is considerable.
Where composting is used, this is relatively costly in terms of energy used and supplies fuel with a relatively low calorific value, e.g. 500 btu maximum. Also where composted fuel is not compact, it is suitable for industrial application only, where the low calorific value is a disadvantage.